1. Field of the Invention
This invention relates to an opaque, biaxially oriented polymer film and a method for its preparation. In particular, it relates to an opaque, biaxially oriented film containing voids and its preparation from a composition comprising a film-forming polymer and void-forming microspheres dispersed therein.
2. Information Disclosure Statement Including Description of Related Art
The following information is disclosed in accordance with the terms of 37 CFR 1.56, 1.97 and 1.98.
U.S. Pat. No. 4,377,616, issued Mar. 22, 1983 to Ashcraft et al., discloses opaque, biaxially oriented polymer film, e.g., of isotactic polypropylene, comprising a core matrix having a stratum of voids containing spherical void-initiating solid particles. The void-initiating particles can be composed of an organic polymer such as a polyamide, e.g.. nylon-6, a polyester, e.g., polyethylene terephthalate acetals, or an acrylic resin, or an inorganic material such as glass, metal or ceramic.
U.S. Pat. No. 4,632,869, issued Dec. 30, 1986 to Park et al., discloses the use of polybutylene terephthalate (PBT) spherical particles as a cavitating agent for polymers such as polypropylene in the formation of opaque, biaxially oriented films. The patentees state that PBT yields certain advantages over previous cavitating agents such as nylon, e.g., its moisture sorption is lower.
U.S. Pat. No. 4,720,416, issued Jan. 19, 1988 to Duncan, discloses a pressure-sensitive label stock material having an opaque, biaxially oriented polymer film facing layer with a core layer containing voids enclosing void-initiating particles. Such particles may be composed of an organic polymer such as polybutylene terephthalate (PBT), a polyethylene terephthalate acetal, a polyamide, or an acrylic resin, or an inorganic material such as glass, metal or ceramic. The patentees state that PBT is an especially preferred void-initiating material because of its good mechanical, chemical and electrical properties. Only PBT void-initiating particles are shown in a working example.
U.S. Pat. No. 4,741,950, issued Mar. 3, 1988 to Liu et al., discloses a surface-treated opaque laminate film comprising a core layer with voids containing void-initiating particles and two skin layers of enhanced wettability. The void-initiating particles may be of a thermoplastic resin such as PBT, polyethylene terephthalate, a polyamide, or an acrylic resin, or an inorganic material such as glass, metal or ceramic. PBT is stated to be particularly preferred as a void-initiating material because of its good mechanical, chemical and electrical properties, and is the only such material shown in a working example under the invention.
U.S. Pat. No. 4,758,462, issued Jul. 19, 1988 to Park et al., discloses an opaque, biaxially oriented film structure comprising an expanded thermoplastic polymer matrix core layer within which is located a light-absorbing pigment and a stratum of voids containing void-initiating particles. Such particles may be composed of a thermoplastic resin, e.g., a polyester such as PBT or polyethylene terephthalate, a polycarbonate, a polysulfone, an acetal, an acrylic resin, or nylon, or an inorganic material, e.g., glass, metal or ceramic.
None of the foregoing U.S. patents disclose the use of cross-linked polystyrene as a void-initiating agent.
D. Zou et al., "Model Filled Polymers I. Synthesis of Crosslinked Monodisperse Polystyrene Beads," Journal of Polymer Science: Part A: Polymer Chemistry.Vol. 28, 1909-1921 (1990), disclose methods of preparation of cross-linked polystyrene beads and states that filled polymer systems are of great economic importance.
C. M. Tseng et al., "Uniform Polymer Particles by Dispersion Polymerization in Alcohol," Polymeric Materials Science and Engineering, Proceedings of the ACS Division of Polymeric Materials: Science and Engineering, Vol. 54, Spring Meeting 1986, New York, N.Y. (American Chemical Society, 1986), 362-366, disclose the preparation of uniform polystyrene particles in the 1-10 micron range by dispersion polymerization in ethyl alcohol.
Neither of the foregoing two references teach that the polystyrene disclosed could be used as a void-initiating agent in the formation of opaque, oriented films.